The detachment of thick films from a substrate is performed particularly in the semiconductor industry. Various detachment processes are already known and widely employed. Certain processes comprise the formation, within the substrate, of a weakened zone defining the film to be detached followed by the application of stresses to the substrate or to the weakened zone so as to cleave the substrate in the weakened zone. These stresses may be of thermal origin and/or mechanical origin, etc.
One particular process is known as the SmartCut™ layer transfer process, which comprises the formation, in a donor substrate, of a weakened zone by implanting atomic species, defining the film to be transferred, followed by the bonding of the donor substrate to a receiver substrate and the cleavage of the donor substrate in the weakened zone, resulting in the film being transferred onto the receiver substrate. This process is more appropriate for transferring thin films, that is to say films or usefully layers typically having a thickness of less than 1 micron.
Industrial implanters currently are designed to provide an energy of up to 200 keV, thereby achieving an implantation depth of possibly up to about 2 microns, depending on the materials and the ionic species implanted. For example, by implanting H+ ions into silicon it is possible to achieve a depth of about 1.8 microns, but the depth to be achieved will be smaller in other substrates, such as GaN, which is denser.
There are high-energy implanters, i.e., operating at around 1 MeV, which would allow a depth of up to 20 microns to be achieved, but the process is not economically viable because of the high cost of these machines. Accordingly, there is a need for lower cost alternatives for implanting at greater depths in the substrate.
US Patent application publication 2007/0249140 describes a method of detaching a layer from a silicon substrate, in which a metal layer, in particular a silver and/or aluminium paste, is deposited on the surface of the substrate. Applying a thermal stress to the substrate covered with the metal layer generates a cleavage stress in the substrate at a depth corresponding to the thickness of the layer that is intended to be detached, resulting in the cleavage of the substrate and detachment of the desired layer.
The cleavage stress obtained depends on the difference in thermal expansion coefficient (TEC) between the material of the metal layer and the material of the substrate. Thus, for example, thermal expansion coefficient of silicon is 4.6×10−6K−1 whereas that of silver is 20×10−6 K−1 and that of aluminium is 24×10−6 K−1.
Depositing metals on the silicon substrates runs the risk of contaminating the semiconductor layer, however, and this would be prejudicial to the operation of the device fabricated from this layer. Moreover, thermal expansion coefficient of the stress-generating layer depends on the nature of the material employed. As a result, a person skilled in the art is necessarily limited in the definition of the process by the materials that are commercially available. In particular, he may necessarily be unable to form a stress-generating layer having the desired thermal expansion coefficient.
Finally, to make the stress-generating layer expand according to the aforementioned method, it is generally necessary to heat it to a high temperature such as typically around 800° C. In this temperature range, however, silicon is ductile, making it unfavourable for cleaving the substrate. Thus, there is a need for a layer transfer process that can detach a film from a substrate when the film typically having a thickness of between 1 and 100 microns, wherein the process avoids the drawbacks of the abovementioned prior art methods. The present invention now provides such a process.